Earth Abundant Silicon Composites as the Electrocatalytic Counter

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Earth Abundant Silicon Composites as the ElectroCatalytic Counter Electrodes for Dye-Sensitized Solar Cells Chun-Ting Li, Yu-Lin Tsai, and Kuo-Chuan Ho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12423 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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ACS Applied Materials & Interfaces

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Earth Abundant Silicon Composites as the Electro-

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Catalytic Counter Electrodes for Dye-Sensitized

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Solar Cells

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Chun-Ting Li1, Yu-Lin Tsai1, and Kuo-Chuan Ho1,2,*

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1

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan

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2

Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

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ABSTRACT: Earth abundant silicon compounds, including Si3N4, SiO2, SiS2, and SiSe2, were

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introduced as the electro-catalytic materials for the counter electrodes (CE) in dye-sensitized solar

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cells (DSSCs). Among these silicon-based materials, Si3N4, SiS2, and SiSe2 were applied in DSSCs

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for

the

first

time.

In

the

presence

of

a

conducting

binder,

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poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), various silicon-based

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composites (Si3N4/PEDOT:PSS, SiO2/PEDOT:PSS, SiS2/PEDOT:PSS, and SiSe2/PEDOT:PSS)

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were successfully coated on the ITO substrates via a simple drop-coating process. In a composite

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film, silicon-based nanoparticles provided attractive electro-catalytic ability and plenty of electro-

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catalytic active sites for the triiodine ion (I3-) reduction. PEDOT:PSS not only acted as a good

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conducting binder for silicon-based nanoparticles, but also provided a continuous polymer matrix

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to increase the electron transfer pathways. By adjusting the weight percent (1 to 5 wt%) of the

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silicon-based nanoparticles (Si3N4, SiO2, SiS2, and SiSe2) with respect to the weight of

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PEDOT:PSS, the composite films containing 5 wt% Si3N4 (denoted as Si3N4-5) and 5 wt% SiSe2

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(denoted as SiSe2-5) both reached excellent electro-catalytic abilities and rendered the good cell

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efficiencies (η) of 8.2% to their DSSCs. It can be said that both Si3N4-5 and SiSe2-5 are promising

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electro-catalytic materials to replace the rare and expensive Pt (η=8.5%).

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KEYWORDS: Counter electrode, Dye–sensitized solar cell, Silicon nitride, Silicon oxide, Silicon

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selenide, Silicon sulfide, Rotating disk electrode

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

INTRODUCTION

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In a dye–sensitized solar cell (DSSC), the counter electrode (CE) functions as the electro-

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catalytic layer to regenerate the redox species. Efficient CEs often possess good electro-catalytic

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ability, high conductivity, large effective surface area, and good stability. Generally, platinum (Pt)

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worked well as the CE. However, several disadvantages of Pt film limited the development of

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DSSCs: (1) Pt CE was easily poisoned by the redox species; (2) Pt CE often lacked of surface area;

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(3) Pt CE required an expensive vacuum fabrication process1-4. Therefore, plenty of electro-

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catalytic materials were explored to replace Pt, including alloy5-7, carbons8-9, conducting

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polymers10-12, transition metal compounds13, and their composites14-15. Among these materials,

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transition metal compounds and the corresponding composites have caught a lot of attention,

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because they owned several advantages, i.e. low-cost, good conductivity, and quite stable.

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Transition metal compounds, e.g., titanium (Ti)16-18, vanadium (V)19-20, chromium (Cr)13,

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zirconium (Zr)13, 21, niobium (Nb)17, 22-23, molybdenum (Mo)24-27, hafnium (Hf)28, tantalum (Ta)29-


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, tungsten (W)24-26, 31-32, iron (Fe)33-36, cobalt (Co)37-38, nickel (Ni)38-39, tin (Sn)40-42, copper (Cu)43-

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, bismuth (Bi)45 compounds etc., have been reported as the potential CEs in DSSCs. However,

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some of them are rare on earth and are toxic to environment.

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Therefore, a group of silicon-based compounds is newly introduced, because silicon is the

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cheapest, the most abundant, and the most eco-friendly semi-metal on earth. For example, Yun et

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al. applied bare silicon carbide (SiC) and Pt decorated SiC film as the CEs, and the pertinent

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DSSCs provided interesting cell efficiencies (η’s) of 3.29% and 7.07%, respectively46. In their

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work, a DSSC with bare SiC CE showed a high open–circuit voltage (Voc) of 0.71 mV and a good

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short–circuit current density (Jsc) of 12.56 mA cm-2, but a very low fill factor (FF) of 0.37; this

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low FF implied the poor contact between the SiC film and the conducting substrate. They further

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dip-coated Pt nanoparticles onto the SiC film to form a SiC/Pt composite CE, and thus the electro-

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catalytic ability of the CE and pertinent cell efficiency were both largely improved. Zhong et al.

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designed a mesoporous titanium carbide/silicon carbide/carbon (TiC/SiC/C) composite CE47. By

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involving silicon carbide into the titanium carbide crystal, the average particles size and pore size

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of the composite film were significantly reduced, and the porosity of the composite film was

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increased. The best TiC/SiC/C composite CE gave its DSSC an η of 5.70%. Wu et al. applied good

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electro-catalysts of molybdenum silicide (MoSi2) and Pt decorated MoSi2 film as the CEs; the

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pertinent DSSCs provided the good η’s of 7.77% and 7.94%, respectively48. Since silicide

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belonged to the interstitial compound, the insertion of the exotic atom (Si) bought many defects to

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the MoSi2 crystalline, as compared to the pure bulk metal (Mo). Accordingly, the inserted defects

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in MoSi2 were considered as the electro-active sites. Song et al. combined pure Si nanoparticles

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and silicon dioxide (SiO2) with poly(3,4ethylenedioxythiophene):poly(styrenesulfonate)

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(PEDOT:PSS); the cells with the CEs of Si/PEDOT:PSS and SiO2/PEDOT:PSS reached the η’s of


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5.70% and 5.66%, respectively49-50. Besides, the composite film of SiO2/PEDOT:PSS was quite

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transparent, and it may be suitable for fabricating bifacial DSSCs. Tsai et al. introduced SiC

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nanoparticles/PEDOT:PSS composite film, which rendered its DSSC a good η of 7.25%, because

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SiC nanoparticles/PEDOT:PSS composite film exhibited good intrinsic heterogeneous rate

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constant and large effective electro-catalytic surface area51.

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To summarize the literatures mentioned above, even though plenty of the transition metal

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compounds were studied as the replacements of the traditional Pt CE, several issues still existed,

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e.g., (1) rare heavy metals were often included, (2) toxic precursors were inevitable, and (3) high

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costs were needed for the fabrication process of transition metal compounds. Therefore, three kinds

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of silicon-based compounds (SiC, SiO2, and MoSi2) were used as the CEs recently due to the fact

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that silicon is the cheapest, non-toxic, and the most earth abundant semi-metal. Up to date, the

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DSSCs with silicon-based CEs only reached the cell efficiencies ~ 7.8%46-51. In order to explore

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more efficient silicon-based electro-catalysts, we systematically investigated four kinds of the

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silicon compounds (Si3N4, SiO2, SiS2, and SiSe2) to form different composite films as the electro-

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catalytic CEs in DSSCs. Among them, Si3N4, SiS2, and SiSe2 were applied in DSSCs for the first

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time. When Si3N4, SiO2, SiS2, and SiSe2 nanoparticles were combined with the conducting binder,

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namely, PEDOT:PSS, four types of the composite films, including Si3N4/PEDOT:PSS,

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SiO2/PEDOT:PSS, SiS2/PEDOT:PSS, and SiSe2/PEDOT:PSS, respectively, were prepared via a

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simple and low-cost drop-coating method. Here, the earth abundant silicon-based nanoparticles

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played an important role in catalyzing the I3- reduction (Scheme 1), while PEDOT:PSS functioned

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as a dispersant and a conductive binder simultaneously. Therefore, PEDOT:PSS can facilitate the

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adhesion between the composite film and the ITO substrate, enhance the linkage among the added

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nanoparticles (i.e. Si3N4, SiS2, SiSe2), and prevent the aggregation of the added nanoparticles52-55.


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This work firstly introduced Si3N4, SiS2, and SiSe2 as the electro-catalysts for I3– reduction,

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systematically compared their performance as the CEs in DSSCs, and reached the highest η record

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up to 8.20% among the DSSCs with silicon-based CEs. Last but not the least, the earth abundant

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Si3N4 (8.18±0.02%) and SiSe2 (8.20±0.03%) can be considered as the promising substitutions for

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Pt (8.50±0.01%). Besides, the simple, low-cost, low temperature, and non-vacuum drop-coating

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process used in this work was compatible for variable substrates and large-scale production, which

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benefited the future industrialization of DSSCs.

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

EXPERIMENTAL DETAILS

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Preparation of the electro-catalytic films as the counter electrodes. A 30 nm–thick Pt film

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was obtained on a tin–doped indium oxide conducting glasses (ITO, UR–ITO007–0.7mm, 5 Ω sq.–

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1

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ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film was fabricated on ITO by a

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simple drop–coating technique using 50 μL PEDOT:PSS mixture (ethanol/PEDOT:PSS/DMSO =

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20/19/1 by volume); the PEDOT:PSS aqueous precursor of PH1000 was obtained from Heraeus.

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Four earth abundant silicon compounds, including silicon nitride (Si3N4, SiO2. Accordingly, the intrinsic electro-catalytic abilities for these four

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kinds of silicon-based compounds may follow the same order. The DSSCs with Si3N4-5 and SiSe2-

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5 CEs have the comparable cell efficiency (8.18% and 8.20%, respectively) to that of the cell with

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Pt CE (8.50%); it is notable that the Si3N4-5 and SiSe2-5 electro-catalytic films possess the great

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potential to replace Pt.

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The incident photon–to–current conversion efficiency (IPCE) curves of the DSSCs with various

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electro-catalytic CEs were measured at the short-circuit condition of each cell in the wavelength

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range of 400-800 nm, as shown in Figure 2b. Except for the cell with PEDOT:PSS CE, other

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DSSCs (including the cells with Pt, Si3N4-5, SiO2-1, SiS2-5, and SiSe2-5 CEs) gave good IPCE

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values around 80~90% from 400 to 600 nm, indicating that those CEs provided fast and sufficient

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electrolyte regeneration. In the case of PEDOT:PSS-based DSSC, its IPCE values were obvious

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lower than the values of other cells; it may be due to that PEDOT:PSS film provided weak electro-

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catalytic ability for I3- and thereby limited the IPCE of the pertinent cell. Besides, each IPCE curve

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was further integrated to get the corresponding short-circuit current density (Jsc-IPCE) value. As

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summarized in Table 1, the Jsc-IPCE values agreed well with the Jsc values obtained from the J–V

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curves. It is notable that the best IPCE value (89%) and Jsc-IPCE value (16.94 mA cm-2) are obtained

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for the DSSC with SiSe2-5 CE; these values are even higher than those of the DSSC with Pt CE.


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Cyclic voltammetry. Cyclic voltammetry (CV) was applied to investigate the redox kinetics of

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iodide/triiodide (I–/I3–) on the surface of an electro-catalytic film. An electrolyte, containing 10.0

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mM LiI, 1.0 mM I2, and 0.1 M lithium perchlorate (LiClO4) in ACN, was used for CV analysis;

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the pertinent analytic details for CV analysis were described in Supporting Information. In a DSSC,

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triiodide ions (I3–) are largely generated, after the iodide ions (I–) trigger the dye regeneration. The

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just-in-time consumption of I3– ions plays a very important role to prevent the recombination and

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the pertinent energy loss of a cell. As a CE in a DSSC, the electro-catalytic film is specifically

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used to facilitate the reduction of I3- at the CEs/electrolyte interface as shown in Equation (1)3.

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I3- +2e - →3I -

(1)

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Generally, the cathodic peak current density (Jpc) of a CV curve indicates the overall electro-

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catalytic ability of a film for I3– reduction, and the larger value of Jpc reflects the better overall

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electro-catalytic ability. CV curves of various electro-catalytic films, i.e., Pt, PEDOT:PSS, Si3N4-

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5, SiO2-1, SiS2-5, and SiSe2-5, were shown in Figure 3; the corresponding Jpc values were 1.39,

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0.40, 1.30, 0.75, 0.99, and 1.38 mA cm-2, respectively (Table 2). Obviously, all silicon-based

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composite films provided higher Jpc values than that of PEDOT:PSS film. It can be confirmed that

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the electro-catalytic abilities of those silicon-based composite films were mainly determined by

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these silicon-based nanoparticles, i.e., Si3N4, SiO2-1, SiS2, and SiSe2, because those silicon-based

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nanoparticles rendered the good intrinsic electro-catalytic capabilities and large surface areas to

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their composite films (Scheme 1). Besides, the Jpc values of the silicon-based composite films

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showed a tendency of SiSe2-5 ≈ Si3N4-5 > SiS2-5 > SiO2-1, which agreed well with the tendencies

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of both Jsc and η values of the pertinent DSSCs. Among all composite films, SiSe2-5 and Si3N4-5

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films gave the highest Jpc values of 1.38 and 1.30 mA cm-2; those high Jpc values were comparable


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with the Jpc value of Pt (1.39 mA cm-2). It was concluded that the SiSe2-5 and Si3N4-5 films have

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the comparable electro-catalytic ability as compared to Pt.

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On the other hand, the cathodic overpotential (ηc) and the peak-to-peak separation potential (ΔEp)

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of a CV curve are both inversely proportional to the intrinsic redox capability of a film. A smaller

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ηc of a film means that a lower cathodic overpotential is required to trigger I3– reduction, while a

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smaller ΔEp of a film indicates a lower overall potential to trigger I–/I3– redox reaction. Therefore,

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the smaller ηc and ΔEp indicate the better intrinsic redox capability of a film toward I–/I3– redox

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reaction. As summarized in Table 2, the ηc values of Pt, PEDOT:PSS, Si3N4-5, SiO2-1, SiS2-5,

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and SiSe2-5 were calculated to be 0.20, 0.33, 0.26, 0.30, 0.27, and 0.26 V, respectively. The ΔEp

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values of Pt, PEDOT:PSS, Si3N4-5, SiO2-1, SiS2-5, and SiSe2-5 were calculated to be 0.40, 0.66,

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0.53, 0.59, 0.54, and 0.52 V, respectively. Since all silicon-based composite films exhibited

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smaller ηc and ΔEp values than those of the PEDOT:PSS film; thus, it was once again verified that

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the electro-catalytic abilities of these silicon-based composite films were attributed to the silicon-

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based nanoparticles. The ηc and ΔEp values showed the same tendency of Pt > SiSe2-5 = Si3N4-5

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< SiS2-5 < SiO2-1 < PEDOT:PSS; this inferred that SiSe2-5 and Si3N4-5 films exhibited the

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comparable intrinsic redox capability with that of Pt.

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Rotating disk electrode. Rotating disk electrode (RDE) technique was used to further

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investigate the electro-catalytic abilities of the films in detail. An electrolyte, containing 1.0 mM

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tetrabutylammonium triiodide (TBAI3) and 0.1 M LiClO4 in ACN, was used for RDE analysis. In

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the case of PEDOT:PSS, Si3N4-5, SiO2-1, SiS2-5, and SiSe2-5, these electro-catalytic films were

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individually drop-coated on a glassy carbon electrode (GCE, with an active area of 0.20 cm2) by


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using 10 μL of their precursor slurries mentioned previously in “EXPERIMENTAL DETAILS.”

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In the case of Pt, another electrode with Pt foil as the disk material (Pt–RDE, with an active area

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of 0.20 cm2) was used for measuring the parameters related to Pt electrode. Thus, the Pt-RDE and

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various GCEs were used as the working electrodes for RDE analysis; other pertinent analytic

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details for RDE analysis are described in Supporting Information. From the CV analysis mentioned

6

above, the Jpc value represents the overall electro-catalytic ability, which can be considered as the

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combination of two key parameters: the intrinsic heterogeneous rate constant (k0) and the effective

8

catalytic surface area (Ae). Therefore, RDE analysis aims to distinguish k0 and Ae simultaneously.

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For an electro-catalytic film, seven linear sweep voltammetry (LSV) curves (not shown) were

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measured under different rotating speeds (50, 100, 200, 400, 600, 800, and 1000 rpm); accordingly,

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seven values of the reciprocal currents (i-1) were obtained from the LSV curves at the formal

12

potential (E0’) based on an electro-catalytic film. The E0’ for an electro-catalytic film is obtained

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from its previous CV curve. Thus, plots of reciprocal current (i-1) vs. reciprocal of rotating rate

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root (ω-0.5) for various electro-catalytic films, including Pt, PEDOT:PSS, Si3N4-5, SiO2-1, SiS2-5,

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and SiSe2-5, were shown in Figure 4. These plots were used for calculating the values of k0 and

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Ae by using a simplified Koutecký–Levich equation, as shown as Equation (2) below57.

17

1

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where i is the specific current obtained from the LSV curve at the formal potential (E0’), n is the

19

number of electrons transferred, F is Faraday constant, C is the concentration of I3- ions, D is the

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apparent diffusion coefficient of I3-, ν is the kinematic viscosity of electrolyte, and ω is the angular

21

velocity converted from the rotating speed.

i

1

1

e

0.62nFAe D2 /3 υ-1 /6 ω1/2 C

= nFA 𝑘 0 C +

(2)


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In Figure 4, the intercept and slope of a fitting line were respectively used to find k0 and Ae

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values of an electro-catalytic film. As summarized in Table 2, the Pt film showed the highest k0

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of 6.70×10-3 cm s-1 and Ae of 0.20 cm2; it indicated the excellent intrinsic electro-catalytic

4

capability of Pt and the limited effective electro-catalytic surface area of the flat Pt film,

5

respectively. PEDOT:PSS film gave the lowest k0 of 1.98×10-3 cm s-1 and comparable Ae of 0.23

6

cm2 to that of Pt; it revealed that PEDOT:PSS has the weakest electro-catalytic ability than the

7

other films. All silicon-based composite films provide both higher k0 and Ae values than those of

8

PEDOT:PSS film. Therefore, it was confirmed that the silicon-based nanoparticles, i.e., Si3N4,

9

SiO2, SiS2, and SiSe2, played the main role to contribute not only the good intrinsic heterogeneous

10

rate constants but also the large effective surface areas to their composite films. Among all silicon-

11

based composite films, their k0 values show a tendency of SiSe2-5 ≈ Si3N4-5 > SiS2-5 > SiO2-1,

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which is consistent with the tendency of the ηc and ΔEp values obtained from CV and the η values

13

of the pertinent DSSCs. Besides, all silicon-based composite films showed comparable Ae values

14

about ≈ 0.7 cm2. Since all silicon-based composite films were coated on the same glassy carbon

15

electrode (GCE) electrode with a fixed geometric area of 0.20 cm2, it can be said that all the

16

composite films were able to provide a rough and porous surface to achieve higher effective

17

surface areas. The Ae values of the composite films were all with 3~3.5 times higher than those of

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Pt and PEDOT:PSS films, indicating those composites provided sufficient surface areas for I3-

19

reduction. As compared to the traditional Pt film, the best two films, namely SiSe2-5 and Si3N4-5,

20

possessed the lower k0 values and the higher Ae values (~3.5 times to that of Pt). Therefore, the

21

overall electro-catalytic abilities of SiSe2-5 and Si3N4-5 films were comparable to Pt film; this

22

RDE showed highly consistency with the above-mentioned CV results.

23


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1

Tafel polarization curves. Tafel polarization technique aimed to explore the practical electro-

2

catalytic abilities of various films for I3– reduction in an electrolyte containing high I–/I3–

3

concentration, i.e., 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M tBP in ACN/MPN (the

4

electrolyte used for the DSSCs), compared to the CV and RDE data collected using the electrolytes

5

with low I–/I3– concentrations. For an electro-catalytic film, a linear sweep voltammetry (LSV)

6

curve was obtained at a low scan rate of 50 mV s-1 by using a symmetric cell composed of the

7

same film on the both sides; and the pertinent Tafel polarization curve was presented as a

8

logarithmic current density–voltage (Log J–V) curve. Generally, a Tafel curve can be divided into

9

three zones: (1) the polarization zone (|V| < 120 mV), (2) the Tafel zone (120 mV< |V| < 400 mV),

10

and (3) the diffusion zone (|V| > 400 mV)57. In the Tafel zone, the exchange current density (J0)

11

of an electro-catalytic film was obtained by extrapolating the anodic and cathodic curves and

12

reading the cross point at 0 V. The higher value of J0 indicates the better electro-catalytic ability

13

of the film. According to the Tafel polarization curves shown in Figure 5, the J0 values for various

14

electro-catalytic films, including Pt, PEDOT:PSS, Si3N4-5, SiO2-1, SiS2-5, and SiSe2-5, were

15

obtained and summarized in Table 2. It can be observed that the J0 values of those films are

16

perfectly consistent with their Jpc values (Pt > SiSe2-5 > Si3N4-5 > SiS2-5 > SiO2-1 > PEDOT:PSS).

17

Therefore, the electro-catalytic abilities of the films were thus validated not only by CV and RDE

18

analysis (with lower I–/I3– concentration) but also by Tafel curve (with higher I–/I3– concentration).

19

As compared to other silicon-based films, the best two films, i.e., SiSe2-5 and Si3N4-5, possessed

20

the highest J0 values of 6.28 mA cm-2 and 5.75 mA cm-2, respectively. Those high J0 values are

21

close to that of Pt, implying their competitive electro-catalytic abilities to Pt.

22 23

Moreover, the J0 value of a film is used to calculate the charge transfer resistance (Rct-Tafel) corresponding to the electro-catalytic film/electrolyte interface, via Equation (3)3, 57.


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RT nFRct -Tafel

Page 16 of 40

1

J0 

2

where R is the ideal gas constant, T is the absolute temperature, F is Faraday constant, and n is the

3

number of electrons transferred for I3– reduction. In general, the smaller Rct-Tafel value refers to the

4

larger amount of electrons transferring through the electro-catalytic film/electrolyte interface, and

5

thereby implies the faster electron transfer capability of the film. In Table 2, all silicon-based

6

composite films provided much lower Rct-Tafel values than that of PEDOT:PSS film, because those

7

silicon-based nanoparticles exhibited higher intrinsic heterogeneous rate constants and larger

8

effective catalytic surface areas than those of PEDOT:PSS. Therefore, the much higher values of

9

Jsc and FF were both obtained from the DSSCs with silicon-based composite CEs. As compared

10

to other silicon-based films, the best two films, i.e., SiSe2-5 and Si3N4-5, possessed the lowest Rct-

11

Tafel

12

that of Pt film (1.75 Ω cm2); therefore, the SiSe2-5 and Si3N4-5 composite films may have the rapid

13

I3– reduction rate comparable to Pt.

(3)

values of 2.05 and 2.23 Ω cm2, respectively. It is notable that those Rct-Tafel values are close to

14 15

Electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy (EIS)

16

was used for precise examination of the interfacial resistances, namely series resistance (Rs) and

17

charge transfer resistance (Rct-EIS), in a symmetric cell composed of one kind of electro-catalytic

18

film on both sides. An electrolyte, containing 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M tBP

19

in ACN/MPN, was used for this symmetric type EIS analysis; the pertinent analytic details were

20

described in Supporting Information. As shown in Figure 6, this type of EIS spectrum usually

21

shows two semicircles in the frequency range of 10 mHz to 65 kHz2-4. According to the equivalent

22

circuit2-4 shown in the inset of Figure 6, the series resistance (Rs) value equals to onset point of


16

Page 17 of 40

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1

the first semicircle (left-hand side) in the high frequency region, the lower Rs reflects the better

2

ohmic contact between the substrate and an electro-catalytic film. The charge transfer resistance

3

(Rct-EIS) is obtained from the radius of the first semicircle in the middle frequency region; the lower

4

Rct-EIS refers to the larger amount of charge pass through the electro-catalytic film/electrolyte

5

interface. The second semicircle (right-hand side) is owing to the Warburg diffusion resistance of

6

the electrolyte, measured in the low frequency region. As summarized in Table 2, the Rs values of

7

PEDOT:PSS and all silicon-based composite films were about 16 Ω cm2, implying they all had

8

good adhesion to FTO substrates. Pt film gave slightly less Rs value (14.25 Ω cm2) than the others,

9

because the ultra-thin Pt had an excellent adhesion to FTO substrate. On the other hand, the Rct-EIS

10

values of the films showed a tendency of Pt < SiSe2-5 < Si3N4-5 < SiS2-5 < SiO2-1 < PEDOT:PSS,

11

which was consistent with the tendency of Rct-Tafel values obtained from Tafel polarization curves.

12

It is notable that all the Rct values (Rct-Tafel or Rct-EIS), irrespective of their measurement technique,

13

show a perfect consistent with the results of Jpc and J0 values. The Rct-EIS value (3.21 Ω cm2) of

14

SiSe2-5 composite is comparable to that of Pt (2.98 Ω cm2).

15

In brief, according to CV, RDE, Tafel, and EIS analyses, the electro-catalytic abilities of the

16

films follow the order of Pt > SiSe2-5 > Si3N4-5 > SiS2-5 > SiO2-1 > PEDOT:PSS. Among all

17

electro-catalytic films, SiSe2-5 exhibited the best electro-catalytic ability for I3– reduction, and

18

showed a great potential to replace Pt due to its low cost and earth abundance. Besides, it can be

19

deduced from this study that the electro-catalytic abilities of these four kinds of silicon-based

20

materials followed the same order of SiSe2-5 > Si3N4-5 > SiS2-5 > SiO2-1. In accordance with the

21

literatures, the electro-catalytic ability of a transition metal compound was reported to be

22

influenced by many complicated factors, including the band structure30, 58-59, the distribution of the

23

density of state30, 58-59, the surface energy60, the adsorption energy21, 61-64, etc.; the information of


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

1

these factors were often obtained via the density functional theory (DFT) calculation. Until now,

2

there are no literatures that provided these information to simultaneously compare the performance

3

of transition metal nitride (TMN), oxide (TMO), sulfide (TMS), and selenide (TMSe). Thus, at

4

this stage, it is extremely hard to speculate a certain trend of the intrinsic electro-catalytic ability

5

for all the transition metal compounds. Moreover, the synthetic approaches for transition metal

6

compounds were reported to dramatically influence their crystal phases22,

7

geometries22, 29-30, 32, 66-67, inner defects30, 36, 66, and thereby their intrinsic electro-catalytic abilities.

8

Since TMNs, TMO, TMS, and TMSe have not been fully investigated, the comparisons of their

9

electro-catalytic abilities would be based on the highest records of their DSSCs’ cell efficiencies.

10

In the case of comparing TMO and TMN, many TMOs (e.g., TiO213, V2O313, Cr2O313, ZrO213,

11

MoO213, 68, and NiO69-71) exhibited poorer intrinsic electro-catalytic abilities than the pertinent

12

TMNs, while some special TMOs (e.g., NbO222, Nb2O522, TaO29, Ta2O529, TaOx30, WO232, WO332,

13

WO2.7266, W18O4967, Fe2O364, Fe3O433) performed better due to the fact that they possessed multiple

14

coordination geometries. Since pure SiO2 used in this study rarely have other coordination

15

geometry, the poorer intrinsic electro-catalytic ability of SiO2-1 than that of Si3N4-5 seems

16

reasonable. In the case of comparing TMO, TMS, and TMSe, their intrinsic electro-catalytic

17

abilities often showed a tendency of TMSe > TMS > TMO, e.g., MoSe272 > MoS226 > MoO213, 68,

18

FeSe235 > FeS273 > Fe2O364, CoSe38,

19

Accordingly, the same tendency (SiSe2-5 > SiS2-5 > SiO2-1) toward the intrinsic electro-catalytic

20

ability was observed in this study; this was judged by the values of k0 obtained from RDE and the

21

values of ηc and ΔEp obtained from CV.

74

40, 65

, coordination

> CoS37 > Co3O437, and NiSe38-39> NiS75 > NiO70-71.

22 23



CONCLUSIONS


18

Page 19 of 40

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1

Silicon-based composites, including Si3N4/PEDOT:PSS, SiO2/PEDOT:PSS, SiS2/PEDOT:PSS,

2

and SiSe2/PEDOT:PSS, were systematically investigated as the electro-catalytic CE for DSSCs;

3

especially for Si3N4, SiS2, and SiSe2, they were introduced in DSSC field for the first time. After

4

adjusting the amounts of the silicon-based nanoparticles in their composite films individually, the

5

DSSCs with Si3N4-5, SiO2-1, SiS2-5, and SiSe2-5 CEs reached cell efficiencies of 8.18%, 5.98%,

6

7.02% and 8.20%, respectively. It is notable that the Si3N4-5 and SiSe2-5 render good electro-

7

catalytic abilities to their DSSC, and thereby their cell efficiencies are comparable to that of the

8

cell with Pt CE (8.50%). From RDE analysis, all silicon-based composites films provided large

9

effective catalytic surface area (Ae), which were about 3~3.5 times higher than those of Pt and

10

PEDOT:PSS films; they also provided good intrinsic heterogeneous rate constants (k0), which were

11

about 1.5~3 times higher than that of PEDOT:PSS. Therefore, the Jpc values obtained from CV

12

analysis and the J0 values obtained from Tafel analysis demonstrated that the overall electro-

13

catalytic abilities of the films showed a tendency of SiSe2-5 > Si3N4-5 > SiS2-5 > SiO2-1>

14

PEDOT:PSS. We concluded that the electro-catalytic abilities of the composite films were mainly

15

supplied by the silicon-based nanoparticles (i.e., Si3N4, SiO2, SiS2, and SiSe2). From Tafel and EIS

16

analyses, the charge transfer capabilities of the silicon-based composites films were much smaller

17

than that of PEDOT:PSS; it indicates: (1) silicon-based nanoparticles promoted the I3- reduction

18

through the CE/electrolyte interface due to their enhanced k0, and (2) the PEDOT:PSS matrix

19

provided a good conductive matrix to facilitate the charge transfer between the electro-catalytic

20

film and the FTO substrate (Scheme 1). Finally, Si3N4-5 and SiSe2-5 CEs exhibited the lower k0

21

values and the higher Ae values (~3.5 times) than those of Pt, and thereby gave the comparable

22

DSSC efficiencies (8.18% and 8.20%, respectively) to that of the cell with Pt CE (8.50%). Thus,

23

Si3N4 and SiSe2 electro-catalytic materials can be both considered to have a great potential to


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 40

1

replace the traditional expensive Pt CE for DSSCs. Most of all, the earth abundant Si3N4-5 and

2

SiSe2-5 benefit the large-scale and low-cost production of DSSCs.

3 4



5

Supporting Information.

6

The detailed information of all the materials/chemicals used in this study and the fabrication details

7

of the TiO2 photoanode were given. This material is available free of charge via the Internet at

8

http://pubs.acs.org.”

ASSOCIATED CONTENT

9 10



11

Corresponding Author

12

*E–mail: [email protected] (Kuo-Chuan Ho)

AUTHOR INFORMATION

13 14



15

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under

16

grant numbers 102-2221-E-002-186-MY3 and 103-2119-M-007-012.

ACKNOWLEDGMENT

17 18



19

1.

20

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1 2


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Page 32 of 40

1

Table 1. Photovoltaic parameters of the DSSCs with various CEs, measured at 100 mW cm–2 (AM

2

1.5G) light intensity. Voc

Jsc

η

Jsc-IPCE

(V)

(mA cm-2)

(%)

(mA cm-2)

Pt

0.73±0.00

16.87±0.05

0.69±0.00

8.50±0.01

14.49

PEDOT:PSS

0.69±0.00

12.79±0.02

0.33±0.02

2.99±0.05

10.28

Si3N4-5

0.76±0.02

16.11±0.01

0.67±0.01

8.18±0.02

13.62

SiO2-1

0.72±0.02

15.05±0.03

0.55±0.01

5.98±0.03

12.36

SiS2-5

0.70±0.00

16.04±0.02

0.63±0.02

7.02±0.06

13.26

SiSe2-5

0.72±0.01

16.98±0.01

0.67±0.01

8.20±0.03

14.94

CE

FF

3 4

Table 2. Electro-catalytic and electrochemical parameters of various CEs. Jpc

ηc

ΔEp

k0 ×10-3

(mA cm-2)

(V)

(V)

(cm s-1)

Pt

1.39

0.20

0.40

6.70

0.20

7.31

1.75

14.25

2.98

PEDOT:PSS

0.40

0.33

0.66

1.98

0.23

0.22

57.51

15.78

-

Si3N4-5

1.30

0.26

0.52

5.87

0.72

5.75

2.23

15.81

4.02

SiO2-1

0.75

0.30

0.59

3.18

0.70

2.45

5.23

16.03

6.83

SiS2-5

0.99

0.27

0.54

5.63

0.62

4.07

3.15

15.96

5.60

SiSe2-5

1.38

0.26

0.52

5.90

0.66

6.28

2.05

15.80

3.21

CE

Ae

J0

Rct-Tafel

Rs

Rct-EIS

(cm2) (mA cm-2) (Ω cm2) (Ω cm2) (Ω cm2)

5 6


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1 2

Scheme 1. The charge transfer sketch of the earth abundant silicon-based composite films.

3 4 5


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Page 34 of 40

1 2

Figure 1. FE–SEM images of the films of (a) Pt, (b) PEDOT:PSS, (c) Si3N4/PEDOT:PSS (Si3N4-

3

5), (d) SiO2/PEDOT:PSS (SiO2-1), (e) SiS2/PEDOT:PSS (SiS2-5), and (f) SiSe2/PEDOT:PSS

4

(SiSe2-5); the inset in (b) is obtained at higher resolution.

5


34

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1

2 3

Figure 2. (a) Photocurrent density–voltage curves and (b) incident photon–to–current conversion

4

efficiency curves of DSSCs with various electro-catalytic films as the CEs. The standard deviation

5

data of each kind of DSSC were obtained using three cells.


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Page 36 of 40

1

2 3

Figure 3. Cyclic voltammograms of various electro-catalytic films, measured at a scan rate of 100

4

mV s-1.

5


36

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1 2

Figure 4. Plots of i–1 vs. ω–0.5 of the RDE with various electro-catalytic films, measured at a scan

3

rate of 2 mV s-1.

4


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Page 38 of 40

1 2

Figure 5. Tafel polarization plot of various electro-catalytic films, measured at a scan rate of 50

3

mV s-1.

4


38

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1 2 3

Fig. 6 Figure 6. Electrochemical impedance spectra of various electro-catalytic films.

4 5


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Page 40 of 40

Table of Content

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3 4 5 6


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Earth Abundant Silicon Composites as the Electrocatalytic Counter

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